This invention relates to the art of geophysical prospecting in general, to the art of radioactivity well logging in particular, and more particularly to a method for determining the true absorption cross section for small quantities of water.
It is known in the art to irradiate the formations for a period of time sufficient to activate an appreciable number of nuclei of material in the formations, thereby rendering the material artifically radioactive, and at an interval of time following the cessation of the radiation to measure the artificial radioactivity. In U.S. Pat. No. 3,379,882, issued to A. H. Youmans and assigned to the assignee of the present invention, there is described one system for radioactivity well logging wherein the formations are irradiated with neutrons from a periodically varying source operating at a repetition rate of the order of magnitude of hundreds or thousands of cycles per second, being thus alternately on and off for periods of hundreds of microseconds. A detector is synchronized with the source to measure returning radiations while the source is off. Such an instrument is used for measuring the absorption cross section (.SIGMA.) of the material surrounding the source and detector. The absorption cross section is a measure of the rate at which thermal neutrons are captured in the material surrounding the instrument.
One use for the above described system has been in log-inject-log. As described in U.S. Pat. No. 3,558,888, issued to A. H. Youmans and assigned to the assignee of the present application, such a logging method can comprise injecting into a subsurface earth formation a fluid having a preselected thermal neutron absorption cross section. Log-inject-log allows one to detect where the fluid is going for the first few feet of formation where the fluid is injected. The solution used by this method is made up by combining with water a known amount of sodium-cloride to provide a salt-water solution for which can be calculated the absorption cross section. It is not unknown to be required to mix more than one quantity of such injection fluid and further it is not uncommon for the absorption cross section to differ slightly from one mixture to the next. One means of controlling the quality of injection fluid would be to measure the absorption cross section of each mixture to determine that the absorption cross sections are constant throughout all the mixtures.
A related problem is encountered in that it has proven to be extremely difficult in the prior art to measure the absorption cross section of very small fluid samples. This difficulty is based on two facts. The absorption cross section of fluids is not based solely on the salinity of the fluid. Impurities, such as trace amounts of boron, can significantly alter the absorption cross section making a measurement unreliable. A second difficulty in determining the absorption cross section of small fluid samples is that there are unique diffusion effects which will alter the neutron decay rate. These diffusion effects will cause to be measured a modified decay rate which is not exactly the true absorption cross section of the fluid samples particularly where small quantities are involved.
These and other disadvantages are overcome with the present invention by providing a method and apparatus for determining the true absorption cross section of small fluid samples based on a unique relationship of various fluid samples as long as the water concentrations of the samples are maintained relative constant.